1. Field of the Invention
The present invention relates to a charging device for charging a photoconductive drum or similar image carrier with a charge roller and a copier, facsimile apparatus, printer or similar image forming apparatus including the same.
2. Description of the Background Art
Generally, an image forming apparatus includes a charging device for charging a photoconductive drum or similar image carrier during an image forming process. While a scorotron charger, corotron charger or similar non-contact type of charging device that does not contact the image carrier has been commonly used, a contact type of charging device is attracting increasing attention because the non-contact type of charging device produces a large amount of undesirable discharge products including ozone. Among some different contact type of charging devices available today, a charging device having a charge roller pressed against the image carrier is extensively used. Japanese Patent Laid-Open Publication No. 2001-337515, for example, proposes a charge roller whose surface is implemented by rubber or resin.
However, a charging device using a charge roller has a problem that toner and impurities accumulate on the surface of the charge roller little by little and make charging irregular, thereby reducing the life of the charging device. To solve this problem, Japanese Patent Laid-Open Publication No. 2001-194868, for example, discloses a charging device in which films, adhered to opposite end portions of a charge roller over the entire circumference, contact an image carrier to thereby form a preselected gap between the center portion of the charge roller and the image carrier. In this configuration, the center portion of the charge roller does not contact the image forming range of the image carrier and is therefore free from the accumulation of smears, so that the life of the charging device is prevented from being reduced. The films, however, start peeling at seams in the circumferential direction of the charge roller due to repeated contact of the charge roller and image carrier.
In light of the above, Japanese Patent Laid-Open Publication No. 2002-55508, for example, teaches a charging device in which elastic, seamless, annular tubes are fitted in annular grooves formed in opposite end portions of a charge roller. The tubes contact an image carrier and form a preselected gap between the center portion of the charge roller and the image carrier, thereby solving the problem particular to the films.
Although tubes or similar annular members are generally thicker and therefore more durable than films, the thickness deviation of each tube in the circumferential direction increases. Therefore, in the charging device using annular tubes, the gap between the center portion of the charge roller and the image carrier is apt to vary due to the thickness deviation to such a degree that the center portion of the charge roller contacts the image carrier. This is particularly true when the photoconductive drum or the body of the charge roller is machined because machining is apt to make the diameter of the drum or that of the roller body larger at the center portion than at the end portions. Further, the drum and charge roller are more likely to contact each other if they are eccentric or nor parallel to each other. Although the charge roller may be machined after the tubes have been fitted thereon, such a procedure is not only time-consuming but also liable to cause the tubes to turn during machining, resulting an increase in cost.
Another problem with the tube scheme is that when the tubes, which are thermally shrinkable and simply fitted in the annular grooves of the charge roller, lose elasticity due to aging, the edges of the tubes get on the circumferential surface of the charge roller and are damaged or increase the gap to thereby bring about abnormal discharge.
Discharge from the charge roller toward the image carrier occurs in the end portions of the charge roller outside of the tubes or similar annular members in the same manner as in the center portion. This causes the image carrier to locally wear little by little and thereby causes a charge bias to leak. In addition, it is likely that toner deposits on the charge roller due to defective cleaning and increases the gap to thereby bring about abnormal discharge.
Laid-Open Publication No. 2001-337515 mentioned earlier shows in FIGS. 10 and 11 a configuration in which the diameter of the charge roller is smaller in the opposite end portions outside of gap forming members in the axial direction than at the center portion. However, this configuration is not directed toward the prevention of discharge in the portions outside of the image forming range, but directed toward easy fitting of the above members that are several millimeters thick and elastic.